Mix the egg yolk and mustard in a tall container. Slowly fold in the oil. Mix with yoghurt. Season to taste with salt and pepper. Wash and peel the celery and cut into fine strips.
Wash and halve apples, cut out the core and also cut into fine strips. Sprinkle with lemon juice. Wash the grapes, dab dry, halve and core. Chop walnuts coarsely if necessary.
Mix all ingredients. Wash the salad and dab dry. Arrange Waldorf salad and green salad. Serve with ciabatta or baguette bread.